Furan and its derivatives are useful precursors for industrial chemicals in the area of, for example, pharmaceuticals, herbicides and polymers. Furan may be converted into tetrahydrofuran (THF) and 1,4-butanediol (1,4-BDO). THF and 1,4-BDO are valuable chemicals used industrially as solvents and in the production of elastic fibres such as elastane/spandex, polybutyrate terephthalate and derivatives of gamma butyrolactone.
These chemicals are usually produced industrially via a number of routes from petrochemical feedstocks, obtainable from fossil fuels. In recent years, increased efforts have focused on producing chemicals, including 1,4-BDO and THF, from renewable feedstocks, such as sugar-based materials.
A method for obtaining furan from non-fossil fuel based sources involves the decarbonylation of furfural. Examples of reaction processes for achieving this and the subsequent conversion of the furan into its derivatives can be found in Hoydonck, H E; Van Rhijn, W M; Van Rhijn, W; De Vos, D E; & Jacobs, P A; (2012) Furfural and Derivatives, in Ullmann's Encyclopedia of Industrial Chemistry (volume 16, pp 285-313), Wiley-VCH Verlag GmBH & Co. KGaA, Weinheim; Dunlop, A P; and Peters, F N; in The Furans Reinhold Publ. Co, 1953; K. J. Zeitsch, in “The Chemistry and Technology of Furfural and its Many By products” Sugar Series 13, Elsevier, 2000; Lange, J-P; van der Heide, E; van Buijtenen, J; and Price, R; Furfural—A Promising Platform for Lignocellulosic Biofuels; ChemSusChem 2012, 5, 150-166 and Watson, J. M.; Ind. Eng. Chem. Prod. Res. Develop., 1973, 12(4), 310. Furfural may be obtained from hemicellulose via acid hydrolysis in the liquid phase as well as in the gas phase as described in WO 2002/22593 and WO 2012/041990.
The conversion of furan to THF and 1,4-BDO by hydrogenation in the presence of water, acetic acid and Raney nickel or oxide supported nickel catalyst is described in Watson, J M; Ind. Eng. Chem. Prod. Res. Develop., 1973, 12(4), 310.
A process for the conversion of furan into 1,4-BDO and THF is taught in U.S. Pat. No. 5,905,159. This document teaches a process in which furan is converted as a reaction mixture with water and in the presence of hydrogen but in the absence of a water-soluble acid in a single stage over a hydrogenation catalyst. The hydrogenation catalyst of U.S. Pat. No. 5,905,159 contains at least one element of subgroup I, V, VI, VII or VIII in the form of a compound or in elemental form, with the restriction that the catalyst does not contain nickel alone being applicable. The preferred catalyst in this process is Re/Ru on active carbon. A similar catalyst is used in the process described in Pan, T; Deng, J; Xu, Q; Zuo, Y; Guo, Q-X and Fu, Y; Catalytic Conversion of Furfural into a 2,5-Furandicarboxylic Acid-based Polyester with Total Carbon Utilisation; ChemSusChem 2013, 6, 47-50.
More effective catalysts for the conversion of furan into 1,4-BDO and THF are taught in co-pending applications EP 14196391.8, said catalysts incorporating rhenium and palladium on solid supports and EP 14199023.4, said catalyst incorporating one or more metals from those in group 8, 9, 10 and 11 of the periodic table supported on amorphous or crystalline aluminosilicate supports.
Heterogeneous, supported catalysts of the types finding use in this area have been found to deactivate over time, often due to the presence of carbonaceous deposits (coking). Catalysts that have experiences such deactivation need to be regenerated again. It is known to regenerate by burning off coke from such deactivated catalysts by using a gas stream comprising oxygen, such as an air stream, at an elevated temperature.
Oxidative catalyst regeneration using air is a cumbersome treatment that requires multiple operation steps, dedicated equipment to feed the reactor with either pure N2 (for purge) or air (for coke burn-off). Further, this requires an accurate reactor monitoring for avoiding runaway during the coke burn-off. By applying oxidative catalyst regeneration there is also the hazard connected with a possible mixing of H2 that may be used for the decarbonylation reaction and O2 needed for the regeneration. A further drawback of oxidative catalyst regeneration is that this cannot be applied to catalysts which comprise carbon as a support, because the carbon support would also be burnt under such oxidative conditions.
It is an object of the present invention to provide a process for regenerating a heterogeneous, supported catalyst used in the production of THF and 1,4-BDO from furan, which process does not have the above drawbacks.